Integrated potentiometric strings are commonly used to realize digital/analog and analog/digital converters. The potentiometric string is used to make discrete a voltage applied to the two end terminals of a chain of resistors connected in series. To realize such converters of up to 8 bits, the embodiment of a resistive potentiometric string commonly comprises 2.sup.n (where n is the number of bits) resistors in series, and, hence, integrated in the form of a unique resistive strip.
When more than 8 bits are required, the linear dimension of the integrated resistance tends to become cumbersome and inconvenient because of the large occupied area of silicon. Above all, it becomes difficult to control essential parameters, such as the differential and integral linearity of the potentiometric string. In these cases, the usual approach is that of subdividing the DAC converter into two structures connected in cascade. A first DAC is realized with a first resistive string to convert the first most significant bits (p). A second resistive string in then connected in cascade to the preceding one to enable the conversion of the remaining least significative bits (m) (p+m=n).
The different known architectures essentially differ in the manner in which the two resistive strings are coupled to each other. FIGS. 1a, 1b and 1c show the most common architectures of resistive strings. These approaches, while overcoming the problems related to the would-be excessive length of a unique integrated potentiometric string, still require a considerable area of integration on the silicon. They also impose heavy burdens for effectively compensating mismatches among the different resistance values.